1. Field of the Invention
The present invention relates to an optical pickup, and more particularly, to an optical pickup capable of performing a high-density recording or reproduction.
2. Description of the Related Art
In general, an optical pickup is adopted to a disk player to record or reproduce information such as image and sound on or from a disk type recording medium. To increase the recording density on a recording medium, the size of a light spot formed on the recording medium must be made as small as possible by an objective lens of an optical pickup. To this end, the optical pickup for recording or reproducing information on or from a digital versatile disk (DVD) adopts an objective lens having a large numerical aperture and a light source of 650 nm wavelength.
Referring to FIG. 1, a conventional optical pickup includes a semiconductor laser 10, a grating 12 to detect a tracking error signal using a 3-beam method, an objective lens 25 for converging an incident light to form a light spot on a recording medium (DVD) 1, a beam splitter 23 for changing an optical path of the incident light, and a photodetector 29 for detecting the incident light and generating a radio frequency (RF) signal and an error signal. The semiconductor laser 10 is a general edge emitting laser for emitting a light of 650 nm.
Also, a collimating lens 21 for collimating the divergent light emitted from the semiconductor laser 10 is placed on an optical path between the semiconductor laser 10 and the objective lens 25. An astigmatism lens 27 is located between the beam splitter 23 and the photodetector 29.
The light emitted from the semiconductor laser 10 passes the beam splitter 23, and is then focused by the objective lens 25 to form a light spot on the recording surface of the recording medium 1. The light reflected from the recording surface of the recording medium 1 travels toward the photodetector 29 via the objective lens 25, the beam splitter 23 and the astigmatism lens 27, and the photodetector 29 generates the RF signal and the error signal.
Here, the size of the light spot formed on the recording surface of the recording medium satisfies the following relationship (1). ##EQU1##
where .lambda. represents the wavelength of the semiconductor laser 10, and NA represents the numerical aperture of the objective lens 25. Thus, in order to achieve high-density recording/reproduction, the size of the light spot must be made small by using the semiconductor laser 10 emitting a short wavelength light, and/or the objective lens 25 having a large numerical aperture.
However, when using the objective lens 25 having a large numerical aperture, aberration caused by the tilt of the recording medium increases, thereby lowering the stability of the light spot. Also, the semiconductor laser 10 capable of emitting a short wavelength light is expensive.
Due to the above reasons, the optical pickup of the DVD adopts an objective lens having a numerical aperture of 0.6 and a semiconductor laser emitting 650 nm wavelength light. In this case, the size of the light spot formed on the recording medium 1 becomes 1.08 .mu.m in diameter. However, it is difficult to record 5 gigabytes of information or more on one side of a recording medium of a 120 mm-diameter disk by using an optical pickup forming such a light spot size.
Thus, the optical pickup cannot be adopted to record or reproduce large quantities of information, as in the case of a high definition (HD) television which requires processing of at least 10 gigabytes of information.